Lithium is the chemical element with atomic number three. Its stable isotopes include lithium-6 and lithium-7. Lithium observed in old stellar atmospheres is an important but processing-sensitive test of Big Bang nucleosynthesis.
The Li3 molecule, with three valence electrons and a bound distorted triangular structure. The equilateral open-shell arrangement is subject to Jahn-Teller coupling; electron counting alone does not establish dissociation energetics.
The Li2 molecule. Its two valence electrons occupy a primarily 2s-derived bonding orbital; filled 1s-derived bonding and antibonding core levels do not give an additional net simple bond-order contribution.
Lithium-7 has three protons and four neutrons. In Big Bang nucleosynthesis it can form directly or through beryllium-7, which later undergoes electron capture. Competition between direct lithium production, lithium destruction and beryllium production produces a valley in the primordial lithium abundance versus baryon-to-photon ratio. Its measured stellar surface abundance need not equal its birth abundance because stellar transport and nuclear burning can remove lithium.
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